In a Plasma Immersion Ion Implantation (PII) process, a semiconductor wafer is placed in a plasma chamber (generally by a wafer handling system), a plasma is ignited, and wafer implantation occurs by pulsing the wafer at a negative potential. This process is repeated for each wafer. A significant challenge associated with PII is related to the repeatability of the process, and notably, one of the primary sources that may introduce variability into the process is related to the plasma ignition phase.
Referring initially to prior art FIG. 1, a conventional PII system 10 is shown. An RF power plasma source (not shown) is generally inductively or capacitively coupled to a plasma chamber 20. Plasma ignition is achieved when sufficient power is injected into the system 10 via an RF antenna 30 (shown as an inductor). Conventionally, power is injected into the system 10 from a fixed frequency (13.56 MHz) RF generator 40 through a 50 ohm coaxial cable 42 via a matching network 50. The matching network 50 is required to provide maximum power to the load by matching the 50 ohm output impedance of the RF generator 40 and a complex impedance established by the power antenna 30 and resultant plasma impedance 60 within the plasma chamber 20. The matching network 50 includes mechanically variable high voltage vacuum capacitors 50a and 50b. The tunable capacitors 50a and 50b account for variations in the antenna impedance caused by changes in plasma impedance 60 before, during and after plasma ignition. Capacitors 50a and 50b are employed to minimize "reflected power" back to the RF generator 40. The reflected power is monitored by a power meter 70, and a reflected power measurement is provided as an input 70a to an RF control 72. Based on the reflected power input 70a, the controller 72 directs a control output 72a to one or more motor drives 74 for adjusting the tunable capacitors 50a and 50b in order to minimize reflected power from the load. It is noted, that if the reflected power becomes too high, the RF generator 40 may fault. An external inductance 76 is depicted between the matching network 50 and the plasma chamber 20 and represents stray inductances associated with the system 10.
Generally, the antenna 30 impedance varies significantly during the plasma ignition phase versus the steady state phase due to the changes caused by the plasma impedance 60. As shown, the plasma impedance 60 may be roughly modeled as a parallel network containing an imaginary component (X) 60a and a real component (R) 60b. During the changes between plasma ignition and steady state, large adjustments of the tuning capacitors 50a and 50b are generally required to account for large values of reflected power due to changes in plasma impedance 60 during ignition. Even though tunability is achieved by capacitors 50a and 50b, the delivered power is often limited to a fraction of the RF generator 40 output capability, and in many cases, plasma ignition is achieved only by increasing the pressure in the plasma source or chamber.
The process of increasing and subsequently reducing pressure, in conjunction with varying the tuning capacitors 50a and 50b, may require more than 10 seconds to complete. This lengthy period of time may enable substantially large voltages to be induced on the antenna 30 and may result in substantial electric fields at the wafer--possibly endangering the devices on the wafer. It is noted that until the plasma is ignited wafers are exposed to the unshielded antenna fields. Furthermore, even before pulsing of the wafer, deposition may occur producing a surface concentration of dopant. Thus, variability in ignition times, source pressures, and voltage transients may result in variations in resultant implant characteristics--making tightly controlled repeatability exceedingly difficult to achieve. Still further, if the control system 72, and/or any of the related circuits 50, 70 and/or 74 fail, the plasma will be lost. Even if the control system 72 performs flawlessly, the system 20 is slow to react and move due to the tuning requirements discussed above.
Another conventional approach to solving the problem of matching a variable impedance plasma source to an RF generator, is by varying the frequency of the generator to maintain a resonant condition. However, this approach also requires a control loop which varies generator frequency to minimize reflected power. The control is generally not fast enough, however, to prevent fault conditions during large and rapid impedance variations as a result of plasma ignition. Thus, power must still be limited. Additionally, this approach generally only matches reactive load changes, and therefore a mechanically variable capacitor may still be required to match resistive load changes.
Consequently, there is a strong need in the art for a system and/or method to provide repeatable and reliable plasma ignition. Moreover, there is a strong need for a PIII system providing a substantially faster, repeatable and more economical plasma ignition process to alleviate the aforementioned problems associated with conventional PIII systems and/or methods.